Since the abrasion resistant property of steel is increased by increasing hardness, steel used for parts which are required to have abrasion resistant property contains C in an amount in accordance with the required hardness and is subjected to a quenching treatment or a quenching and tempering treatment.
When a high-hardness abrasion-resistant steel plate is reheated to temperature region of about 300° C. to 400° C. occurring a low-temperature temper embrittlement as a result of performing, for example, welding, gas cutting, or plasma cutting, cracking may occur due to delayed fracturing after the steel plate has been cooled to room temperature. However, since processing such as welding or gas cutting is indispensable, it is an issue to prevent the above-described cracking. Cracking due to delayed fracturing in a portion which has been reheated to the temperature region occurring a low-temperature temper embrittlement may be referred to as “low-temperature temper embrittlement cracking” or “low-temperature embrittlement cracking” in some cases.
In addition, an abrasion-resistant steel plate may be used in an operation of a low-temperature range of 0° C. or lower, and thus there is a problem of brittle fracturing occurring in use in the case of a low-toughness steel plate. Generally, increasing the amount of C contained in order to increase hardness or adding alloying elements in order to increase hardenability conversely causes a decrease in toughness as a result of the embrittlement of the material. Various techniques have been proposed regarding an abrasion-resistant steel plate.
For example, abrasion-resistant steel plates excellent in delayed fracturing resistance proposed in Patent Literature 1 through Patent Literature 6 are intended to increase the delayed fracturing resistance of a steel plate in the manufactured state without further treatments, and no consideration is given to increasing delayed fracturing resistance in a portion which has been reheated to a temperature in the range in which low-temperature temper embrittlement occurs.
Regarding an abrasion-resistant steel plate excellent in low-temperature toughness, for example, Patent Literature 7, Patent Literature 8, and Patent Literature 9 disclose techniques in which the toughness of an abrasion-resistant steel plate is increased by adding alloying elements such as Cr and Mo in large amounts. In the case of these techniques, Cr is added in order to increase hardenability, and Mo is added in order to increase hardenability and grain boundary strength at the same time. In addition, in Patent Literature 7 and Patent Literature 8, low-temperature toughness is increased by performing a tempering heat treatment.
On the other hand, examples of a technique in which a manufacturing process is devised include one disclosed in Patent Literature 10, and the literature describes that toughness is increased by elongating prior austenite grains through the utilization of ausforming in a hot rolling process. As an example of a technique for inhibiting low-temperature embrittlement cracking, Patent Literature 11 discloses a technique in which martensite is formed as a matrix structure where a prior austenite grain diameter is controlled to be 30 μm or less in order to inhibit cracking and to increase toughness.